1. Field of the Invention
The present invention relates to a method of and an apparatus for welding a workpiece in a manufacturing process such as for manufacturing automobiles.
2. Description of the Related Art
On automobile production lines, a plurality of panel-like workpieces are assembled and welded into an automobile body. For producing such an automobile body, it has heretofore been customary to grip and position workpieces with clamp jigs, and weld the workpieces at a plurality of spots thereon with resistance spot welding machines. Since it is necessary to displace a resistance spot welding machine into contact with workpieces without engaging clamp jigs, the spots where the workpieces can be welded by the resistance spot welding machine are limited by the clamp jigs, possibly resulting in lack of sufficient mechanical strength at some welded regions of the workpieces. One solution to this problem has been to weld workpieces according to a two-stage welding process. In the two-stage welding process, workpieces are provisionally welded such that the welded workpieces will remain assembled in a desired shape after they are removed from the clamp jigs, and then removed from the clamp jigs, after which the workpieces are welded at any desired spots thereon by resistance spot welding machines. In this manner, the welded workpieces have a sufficient level of mechanical strength.
The two-stage welding process, however, increases the number of steps of the automobile manufacturing process, increases the period of time required to produce each automobile, and lowers the efficiency with which automobiles are manufactured. Furthermore, the two-stage welding process is responsible for an increase in the length of automobile production lines and an increase in the number of automobile body carriages, the number of clamp jigs, and the number of spot welding machines. As a consequence, the automobile production lines are relatively costly, making it relatively expensive to manufacture automobiles. Another problem is that if the tip of a welding gun of a welding robot sticks to a workpiece while the workpiece is being spot-welded, the welding gun pulls the workpiece when the welding robot is displaced, tending to deform the workpiece. The spot-welding process is disadvantageous in that the automobile production line needs to be shut down in order to dress the tip of the welding gun, and hence the efficiency of the automobile production line cannot be increased for this reason.
Various laser beam welding apparatus have been proposed in the art to eliminate the difficulties of the spot welding process. For example, Japanese patent publication No. 4-36792 discloses a multipoint welding apparatus having a plurality of welding heads. The welding heads are positioned in the vicinity of workpiece regions to be welded, and a desired one of the welding heads is selected by adjusting the angle of a plane mirror. A laser beam is applied through the selected welding head to the corresponding workpiece region to be welded. In this manner, a plurality of workpiece regions can be welded in a relatively short period of time.
However, the disclosed multipoint welding apparatus suffers various problems. Specifically, since the welding heads are fixed with respect to the workpiece, they can spot-weld the workpiece with a laser beam, but fail to seam-weld the workpiece. Because as many welding heads as the number of workpiece regions to be welded are necessary, the multipoint welding apparatus is highly expensive, and the number of welding heads and their positions must be changed depending on the type of workpieces to be welded. Such a changing process is tedious and time-consuming. In addition, inasmuch as the welding heads are positioned in the vicinity of workpiece regions to be welded, mirrors in the welding heads are liable to be smeared by sputtered materials during the welding process, and hence should be replaced or otherwise serviced frequently.